Display devices which can be selectably illuminated to convey changing information have become increasingly important. The earliest illuminated display devices were fixed-message signs, such as "exit" or "no smoking" signs which contained a source of light capable of being turned on and off, and a partially translucent face plate containing the message. This type of display suffered washout by strong front illumination. U.S. Pat. No. 3,682,531, issued to A. R. Jeffers teaches a light trap consisting of a circular polarizer and optionally also of a specular foraminous screen to trap ambient light entering the face of the sign.
Electronically controllable display devices have grown particularly with the growth of computers. For example, a gas-discharge device containing a stacked set of transparent plates having shaped discharge regions therein has been in use for many years. The shaped discharge regions may for example form the numerals 0 through 9, one to a transparent plate. When the discharge regions in one plate are electrically energized, the characteristic glow of the gas discharge regions forms one of the numerals. Except when the particular plate illuminated is nearest the viewer, the illuminated numeral is viewed through one or more transparent deenergized plates. When an array of these gas-discharge devices are used to display multiple numerals, the varying distance of the illuminated plates from the viewer gives rise to annoying parallax.
In an attempt to eliminate parallax in a gas-discharge display device, S. M. Frouws, in U.S. Pat. No. 3,418,509, disclosed a planar gas discharge tube containing individually energizable segments spaced away from a counter electrode of transparent conductive material or a fine gauge wire screen through which the viewer observed the segments. The gas discharge was set up between the counter electrode and the energized segments. This device suffered, in common with all gas discharge devices, the need for high voltages and inductive current limiting. This made it impractical to directly drive gas-discharge display devices from modern solid state electronics.
The modern development of vacuum fluorescent display has solved the parallax and high-voltage problems of the gas discharge display devices while requiring the solution of a number of problems of its own.
A vacuum fluorescent display device uses a filament, heated to below incandescent temperature, as a source of thermionic electrons which are then accelerated toward an anode coated with a phosphor capable of fluorescing under bombardment by low-energy electrons. The accelerating voltage can be from a few volts to hundreds of volts but is preferably in the range of from 10 to 30 volts. By selectively accelerating thermionic electrons to desired regions of the phosphor-coated anode, a bright changeable planar display is achieved.
It was discovered by R. DuBois that natural electrostatic charges, such as from a comb run through a person's hair and brought into proximity of a vacuum fluorescent display of the type described, could completely extinguish the display for an extended time. His solution, disclosed in U.S. Pat. No. 3,584,252, consisted in partially encircling the rear and side regions of the anode with a conductive electrostatic shield.
A second problem of unequal illumination of the anode by thermoelectrons has engendered a number of solutions. The problem arises because a convenient method of fabrication includes an insulating substrate, usually glass, behind or embedding the anodes. Charges stored in the insulating substrates so distort the electric field within the vacuum fluorescent device that widely variable illumination of the phosphor occurs. Solutions by R. Raago in U.S. Pat. No. 3,780,326 and by S. Shimada in U.S. Pat. No. 3,668,466 taught the use of an auxiliary electrode in substantially the same plane as the anodes. Application of the correct voltages on the auxiliary electrode could adjust the electrostatic field to achieve uniform illumination or alternatively could extinguish the device. R. Raago in U.S. Pat. No. 3,688,147 solved the problem in a different way by spacing the anode segments on cantilevers far enough forward from the insulating substrate to avoid the distortion of the electrostatic field from charges stored in the insulator. Still another solution, disclosed in varying forms by M. Tanji in U.S. Pat. Nos. 3,619,694 and 3,508,101 and by R. DuBois in U.S. Pat. No. 3,566,187 uses a mesh grid interposed between the filament and the anodes operating in a fashion analogous to a normal electron-tube screen grid to accelerate electrons toward the anode using positive voltage or to cut off electron flow to the anode using negative voltage. Proper adjustment of the positive voltage on the grid was effective to cancel the effect of charges stored in the insulating substrate. In addition, the grid shields the anodes from external electrostatic disturbances. The location of the grid between the filament and the anodes fails to protect the filament from disturbances by external electrostatic fields. In addition, the grid, being positive, attracts electrons to itself. Thus a large current, not contributing to display output, is set up with consequent heating and waste of power.